JP6813127B1 - Steel - Google Patents

Abstract

Chemical composition is mass%, C: 0.0010% or more and less than 0.010%, Si: 0.03 to 0.60%, Mn: 0.10 to 1.50%, Cu: 0.05 to 0.50%, Sb: 0.02 to 0.30%, Ni: 0.02 to 0.50 %, Cr: 0.02 to 0.09%, Al: 0.005 to 0.080%, N: 0.008% or less, P: 0.025% or less, S: 0.001 to 0.015%, O: 0.0005 to 0.0035%, Mo: 0 to 0.50%, W 0 to 0.50%, Sn: 0 to 0.30%, As: 0 to 0.30%, Co: 0 to 0.30%, Bi: 0 to 0.010%, Ti: 0 to 0.050%, Nb: 0 to 0.10%, V: 0 to 0.10%, Zr: 0 to 0.050%, Ta: 0 to 0.050%, B: 0 to 0.010%, Ca: 0 to 0.010%, Mg: 0 to 0.010%, REM: 0 to 0.010%, balance: Fe And impurities, CI: 17.0 to 30.0, DI: 1.00 to 3.00, Ceq: 0.100 to 0.220, MnS and MnS oxides are contained in the steel material, the number density of MnS is less than 10.0 / mm2, and MnS A steel material in which the ratio of the number density of MnS oxides to the number density is 0.10 or more.

Description

The present invention relates to steel materials.

Exhaust gas containing steam, sulfur oxides, hydrogen chloride, etc. is generated in the incinerators of boiler fireplaces and waste incinerators. When this exhaust gas is cooled in an exhaust gas chimney or the like, it condenses into sulfuric acid and hydrochloric acid, which causes significant corrosion on the steel materials constituting the exhaust gas flow path, as known as sulfuric acid dew point corrosion and hydrochloric acid dew point corrosion.

To solve such problems, sulfuric acid / hydrochloric acid dew point corrosion resistant steel and highly corrosion resistant stainless steel have been proposed. For example, Patent Documents 1 to 4 propose steel materials having excellent sulfuric acid dew point corrosion resistance to which Cu, Sb, Co, Cr and the like are added. Further, Patent Document 5 proposes a highly corrosion-resistant stainless steel to which Cr, Ni and the like are added.

Japanese Unexamined Patent Publication No. 2001-164335 Japanese Unexamined Patent Publication No. 2003-213367 Japanese Unexamined Patent Publication No. 2007-239094 Japanese Unexamined Patent Publication No. 2012-57221 Japanese Unexamined Patent Publication No. 7-316745

Steel materials containing Cu, Sb, Cr, etc. exhibit excellent corrosion resistance in a sulfuric acid corrosive environment such as an exhaust gas chimney. However, in order to extend the life of boilers and incineration facilities, further improvement in corrosion resistance is expected.

In addition to the exhaust gas chimney, it is also applied to steel materials used for gasification and melting furnaces, heat exchangers, gas-gas heaters, desulfurization equipment, electrostatic collectors, etc., especially steel materials used for heat transfer materials (fin materials). From the viewpoint of property and productivity, not only corrosion resistance but also hot workability and cold workability are required.

An object of the present invention is to solve the above problems and to provide a steel material having excellent corrosion resistance in a sulfuric acid corrosive environment and a hydrochloric acid corrosive environment, and also having excellent hot workability and cold workability.

The present invention has been made to solve the above problems, and the following steel materials are the gist of the present invention.

(1) The chemical composition is mass%
C: 0.0010% or more and less than 0.010%,
Si: 0.03 to 0.60%,
Mn: 0.10 to 1.50%,
Cu: 0.05 to 0.50%,
Sb: 0.02 to 0.30%,
Ni: 0.02 to 0.50%,
Cr: 0.02 to 0.09%,
Al: 0.005 to 0.080%,
N: 0.008% or less,
P: 0.025% or less,
S: 0.001 to 0.015%,
O: 0.0005 to 0.0035%,
Mo: 0-0.50%,
W: 0 to 0.50%,
Sn: 0 to 0.30%,
As: 0-0.30%,
Co: 0-0.30%,
Bi: 0-0.010%,
Ti: 0 to 0.050%,
Nb: 0 to 0.10%,
V: 0 to 0.10%,
Zr: 0 to 0.050%,
Ta: 0 to 0.050%,
B: 0 to 0.010%,
Ca: 0-0.010%,
Mg: 0-0.010%,
REM: 0-0.010%,
Remaining: Fe and impurities,
The CI defined by the following equation (i) is 17.0 to 30.0.
The DI defined by the following equation (ii) is 1.00 to 3.00.
The Ceq defined by the following equation (iii) is 0.1000 to 0.220.
For the number density of MnS containing MnS and MnS oxide in the steel material, the number density of MnS having a maximum length of 2.0 μm or more is less than 10.0 / mm ² , and the maximum length is 2.0 μm or more. The ratio of the number density of MnS oxides having a maximum length of 2.0 μm or more is 0.10 or more.
Steel material.
CI = (Cu / 64) / (S / 32) ... (i)
DI = (Cu / 64) / (Ni / 59) ... (ii)
Ceq = C + Mn / 6 + (Cu + Ni) / 5+ (Cr + Mo + V) / 15 ... (iii)
However, the element symbol in the above formula represents the content (mass%) of each element contained in the steel material, and if it is not contained, 0 is substituted.

(2) The chemical composition is mass%.
Mo: 0.01-0.50%,
W: 0.01 to 0.50%,
Sn: 0.01 to 0.30%,
As: 0.01-0.30%,
Co: 0.01 to 0.30%, and Bi: 0.001 to 0.010%,
Contains one or more selected from
The steel material according to (1) above.

(3) The chemical composition is mass%.
Ti: 0.001 to 0.050%,
Nb: 0.001 to 0.10%,
V: 0.005 to 0.10%,
Zr: 0.005 to 0.050%,
Ta: 0.001 to 0.050%, and B: 0.0003 to 0.010%,
Contains one or more selected from
The steel material according to (1) or (2) above.

(4) The chemical composition is mass%.
Ca: 0.0005 to 0.010%,
Mg: 0.0001 to 0.010%, and REM: 0.0001 to 0.010%,
Contains one or more selected from
The steel material according to any one of (1) to (3) above.

According to the present invention, it is possible to provide a steel material having good corrosion resistance in an acid-corroded environment and excellent in both hot workability and cold workability.

As a result of detailed investigation of the corrosion resistance, hot workability, and cold workability of the steel material in order to solve the above-mentioned problems, the present inventors have obtained the following findings.

From the studies by the present inventors, it has been found that it is effective to generate CuS in the steel in order to improve the corrosion resistance of the steel containing Cu and Sb. Cu and S are required to generate CuS in the steel, but S tends to form inclusions and easily becomes a corrosion origin on the surface of the steel material. Therefore, it was found that the balance between Cu and S is important, and it is necessary to set the value of the acid resistance corrosion index CI defined by the following equation (i) in an appropriate range.
CI = (Cu / 64) / (S / 32) ... (i)

Further, although Cu is effective for corrosion resistance, it causes surface red-hot brittleness, and Cu exceeding the solid solution limit in steel segregates at grain boundaries or the surface of ground iron, causing surface cracking. By containing Ni together with Cu, the surface segregation of Cu is suppressed. That is, by simultaneously containing Cu and Ni, it is possible to prevent surface cracking while maintaining corrosion resistance. Therefore, in order to further study the relationship between Cu and Ni and maintain stable manufacturability, it is necessary to set the value of the red hot embrittlement index DI defined by the following equation (ii) to an appropriate range. It turned out to be necessary.
DI = (Cu / 64) / (Ni / 59) ... (ii)

Furthermore, in order to ensure cold workability while improving corrosion resistance, it is important to set the C content to less than 0.010% and set the Ceq defined by the following equation (iii) to an appropriate range. I got the knowledge.
Ceq = C + Mn / 6 + (Cu + Ni) / 5+ (Cr + Mo + V) / 15 ... (iii)

As described above, in the present invention, since S has an effect of improving corrosion resistance by being contained together with Cu and Sb, extreme reduction is not preferable. On the other hand, when Mn, which is an essential element for ensuring the strength and toughness of steel, forms MnS, the corrosion resistance in an acid-corroded environment is deteriorated.

By appropriately adjusting the Cu content in relation to the S content, many S can be fixed as CuS. However, in particular, the steel material used for fin materials and the like is very thin and easily affected by corrosion, so it is necessary to reduce the mixing of a small amount of MnS as much as possible.

As a result of further studies by the present inventors in order to solve this problem, it has been found that MnS can be made harmless by making it finer and combining it with oxygen to form an MnS oxide.

Further, in the component system having a low C content in the present invention, although the mechanism is not clear, it was found that Cr has an effect of improving sulfuric acid resistance but deteriorates hydrochloric acid resistance. Therefore, it has been found that it is necessary to strictly limit the Cr content in order to achieve both sulfuric acid resistance and hydrochloric acid resistance.

The present invention has been made based on the above findings. Hereinafter, each requirement of the present invention will be described in detail.

(A) Chemical composition The reasons for limiting each element are as follows. In the following description, "%" for the content means "mass%".

C: 0.0010% or more and less than 0.010% C is an element that improves the strength of the steel material. However, in order to ensure cold workability while improving corrosion resistance, it is necessary to reduce the C content as much as possible. Therefore, the C content is set to 0.0010% or more and less than 0.010%. When strength is required, the C content is preferably 0.0030% or more. The C content is preferably 0.0090% or less, more preferably 0.0080% or less.

Si: 0.03 to 0.60%
Si is an element that contributes to deoxidation and improvement of strength and controls the morphology of oxides. However, when Si is excessively contained, the oxide increases and the corrosion resistance is impaired. Therefore, the Si content is set to 0.03 to 0.60%. The Si content is preferably 0.05% or more, more preferably 0.10% or more. The Si content is preferably 0.40% or less, and preferably 0.30% or less.

Mn: 0.10 to 1.50%
Mn is an element that improves strength and toughness. However, when Mn is excessively contained, coarse MnS is generated, and the corrosion resistance and mechanical properties are deteriorated. Therefore, the Mn content is set to 0.10 to 1.50%. The Mn content is preferably 0.30% or more, and more preferably 0.50% or more. The Mn content is preferably 1.20% or less, more preferably 1.00% or less, and even more preferably 0.80% or less.

Cu: 0.05 to 0.50%
Cu is an element that remarkably exhibits corrosion resistance to sulfuric acid and hydrochloric acid when contained at the same time as Sb. However, when Cu is excessively contained, the hot workability is lowered and the productivity is impaired. Therefore, the Cu content is set to 0.05 to 0.50%. The Cu content is preferably 0.10% or more, 0.15% or more, or 0.20% or more. The Cu content is preferably 0.40% or less, more preferably 0.30% or less.

Sb: 0.02 to 0.30%
Sb is an element that remarkably exhibits corrosion resistance to sulfuric acid and hydrochloric acid when contained at the same time as Cu. However, when Sb is excessively contained, the hot workability is lowered and the productivity is impaired. Therefore, the Sb content is set to 0.02 to 0.30%. The Sb content is preferably 0.03% or more, more preferably 0.06% or more, and further preferably 0.10% or more. Further, the Sb content is preferably 0.20% or less, and more preferably 0.15% or less.

Ni: 0.02 to 0.50%
Ni is an element that improves corrosion resistance in an acid-corrosive environment, and in addition, has the effect of improving manufacturability in steel containing Cu. Cu has a great effect of improving corrosion resistance, but it is easily segregated, and when it is contained alone, it may promote cracking after casting. On the other hand, Ni has an effect of reducing the surface segregation of Cu. By containing Ni, in addition to suppressing segregation of Cu and cracking of slabs, the occurrence of local corrosion due to segregation is also suppressed, so that the effect of improving corrosion resistance can be obtained. However, Ni is an expensive element, and its content in a large amount causes an increase in steelmaking cost. Therefore, the Ni content is set to 0.02 to 0.50%. The Ni content is preferably 0.05% or more, more preferably 0.10% or more, and even more preferably 0.15% or more. The Ni content is preferably 0.30% or less, and preferably 0.25% or less.

Cr: 0.02 to 0.09%
Cr is an element having the effect of improving hardenability, strength, and sulfuric acid resistance. However, Cr reduces the hydrochloric acid resistance. Therefore, the Cr content needs to be strictly limited and is set to 0.02 to 0.09%. The Cr content is preferably 0.03% or more, more preferably 0.05% or more, and even more preferably 0.06% or more. Further, the Cr content is preferably 0.08% or less.

Al: 0.005 to 0.080%
Al is added as an antacid. However, when Al is excessively contained, the corrosion resistance is impaired due to the increase in inclusions. Therefore, the Al content is set to 0.005 to 0.080%. The Al content is preferably 0.010% or more, and more preferably 0.020% or more. The Al content is preferably 0.070% or less, more preferably 0.050% or less.

N: 0.008% or less N is an impurity and reduces the mechanical properties and productivity of the steel material. Therefore, the upper limit of the N content is set to 0.008% or less. The N content is preferably 0.006% or less, and more preferably 0.004% or less. The N content may be 0%, but an extreme reduction leads to an increase in steelmaking cost. Therefore, the N content may be 0.001% or more. Further, N has an effect of contributing to improvement of mechanical properties and the like by precipitating as a fine nitride. If the effect is desired, the N content may be 0.002% or more.

P: 0.025% or less P is an impurity and reduces the mechanical properties and productivity of the steel material. Therefore, the upper limit of the P content is set to 0.025% or less. The P content is preferably 0.020% or less, more preferably 0.015% or less. The P content is preferably reduced as much as possible, that is, the content may be 0%, but an extreme reduction leads to an increase in steelmaking cost. Therefore, the P content may be 0.001% or more.

S: 0.001 to 0.015%
S is generally an impurity and reduces the mechanical properties and productivity of the steel material. However, in the present invention, S has an effect of improving corrosion resistance in an acid-corrosive environment by containing Cu and Sb at the same time. Therefore, the S content is set to 0.001 to 0.015%. The S content is preferably 0.003% or more, or 0.005% or more. The S content is preferably 0.013% or less, more preferably 0.011% or less.

O: 0.0005 to 0.0035%
O is an element that has the effect of detoxifying MnS by binding to MnS and preventing deterioration of corrosion resistance and mechanical properties. However, when O is excessively contained, it produces a coarse oxide that is a starting point of corrosion in an acid corrosive environment. Therefore, the O content is set to 0.0005 to 0.0035%. The O content is preferably 0.0010% or more, more preferably 0.0015% or more. The O content is preferably 0.0030% or less, more preferably 0.0025% or less.

In the chemical composition of the steel of the present invention, in addition to the above elements, one or more selected from Mo, W, Sn, As, Co, and Bi in order to improve the corrosion resistance in an acid corrosive environment are described below. It may be contained in the range shown in. Since these elements are not always essential in steel materials, the lower limit of the content is 0%. The reasons for limiting each element will be described.

Mo: 0 to 0.50%
Mo is an element that improves corrosion resistance in an acid-corrosive environment by being contained at the same time as Cu and Sb, and may be contained as necessary. However, since Mo is an expensive element, excessive content causes a decrease in economic efficiency. Therefore, the Mo content is set to 0.50% or less. The Mo content is preferably 0.40% or less, more preferably 0.30% or less, and even more preferably 0.20% or less. When the above effect is desired, the Mo content is preferably 0.01% or more, 0.02% or more, 0.05% or more, or 0.10% or more.

W: 0 to 0.50%
Since W is an element that improves corrosion resistance in an acid-corrosive environment by being contained at the same time as Cu and Sb like Mo, it may be contained as necessary. However, since W is an expensive element, excessive content causes a decrease in economic efficiency. Therefore, the W content is set to 0.50% or less. The W content is preferably 0.40% or less, more preferably 0.30% or less, and even more preferably 0.20% or less. When the above effect is desired, the W content is preferably 0.01% or more, 0.02% or more, 0.05% or more, or 0.10% or more.

Sn: 0 to 0.30%
Since Sn is an element that improves corrosion resistance in an acid-corrosive environment when it is contained at the same time as Cu, it may be contained if necessary. However, if Sn is contained in excess, the hot workability is lowered. Therefore, the Sn content is set to 0.30% or less. The Sn content is preferably 0.25% or less, more preferably 0.20% or less, and even more preferably 0.15% or less. When the above effect is desired, the Sn content is preferably 0.01% or more, more preferably 0.02% or more, and further preferably 0.05% or more. ..

As: 0 to 0.30%
Although As has no significant effect as compared with Sb and Sn, it may be contained as necessary because it is an element effective for improving corrosion resistance in an acid-corroded environment. However, if As is excessively contained, the hot workability is lowered. Therefore, the As content is set to 0.30% or less. The As content is preferably 0.20% or less, and more preferably 0.10% or less. When the above effect is desired, the As content is preferably 0.01% or more, more preferably 0.02% or more, and further preferably 0.05% or more. ..

Co: 0 to 0.30%
Co does not have a remarkable effect as compared with Sb and Sn, but since it is an element that improves corrosion resistance in an acid-corrosive environment, it may be contained if necessary. However, if Co is excessively contained, the economic efficiency is lowered. Therefore, the Co content is set to 0.30% or less. The Co content is preferably 0.20% or less, and more preferably 0.10% or less. When the above effect is desired, the Co content is preferably 0.01% or more, more preferably 0.02% or more, and further preferably 0.05% or more. ..

Bi: 0-0.010%
Bi has no significant effect as compared with Sb and Sn, but since it is an element that improves corrosion resistance in an acid-corroded environment, it may be contained as necessary. However, if Bi is excessively contained, the hot workability is lowered. Therefore, the Bi content is set to 0.010% or less. The Bi content is preferably 0.007% or less, more preferably 0.005% or less. When the above effect is desired, the Bi content is preferably 0.001% or more, more preferably 0.002% or more, and further preferably 0.005% or more. ..

In the chemical composition of the steel of the present invention, in addition to the above elements, one or more selected from Ti, Nb, V, Zr, Ta, and B in order to improve mechanical properties and the like are included in the range shown below. May be contained in. Since these elements are not always essential in steel materials, the lower limit of the content is 0%. The reasons for limiting each element will be described.

Ti: 0 to 0.050%
Since Ti is an element that forms a nitride and contributes to the refinement of crystal grains and the improvement of strength, it may be contained if necessary. However, when Ti is excessively contained, the nitride becomes coarse and the mechanical properties deteriorate. Therefore, the Ti content is set to 0.050% or less. The Ti content is preferably 0.040% or less, more preferably 0.030% or less, and even more preferably 0.020% or less. When the above effect is desired, the Ti content is preferably 0.001% or more, more preferably 0.002% or more, and further preferably 0.005% or more. ..

Nb: 0 to 0.10%
Like Ti, Nb is an element that forms a nitride and contributes to the refinement of crystal grains and the improvement of strength, and therefore may be contained as necessary. However, when Nb is excessively contained, the nitride becomes coarse and the mechanical properties deteriorate. Therefore, the Nb content is set to 0.10% or less. The Nb content is preferably 0.050% or less, more preferably 0.030% or less, and even more preferably 0.020% or less. When the above effect is desired, the Nb content is preferably 0.001% or more, more preferably 0.002% or more, and further preferably 0.005% or more. ..

V: 0 to 0.10%
Like Ti and Nb, V is an element that forms a nitride and contributes to the refinement of crystal grains and the improvement of strength, and therefore may be contained as necessary. However, when V is excessively contained, the nitride becomes coarse and the mechanical properties deteriorate. Therefore, the V content is set to 0.10% or less. The V content is preferably 0.050% or less, more preferably 0.030% or less, and even more preferably 0.020% or less. When the above effect is desired, the V content is preferably 0.005% or more.

Zr: 0 to 0.050%
Like Ti, Nb, and V, Zr is an element that forms a nitride and contributes to the refinement of crystal grains and the improvement of strength, and therefore may be contained as necessary. However, Zr is an expensive element, and a large amount of Zr causes an increase in steelmaking cost. In addition, if Zr is excessively contained, the nitride becomes coarse and the mechanical properties deteriorate. Therefore, the Zr content is set to 0.050% or less. The Zr content is preferably 0.040% or less, more preferably 0.030% or less, and even more preferably 0.020% or less. When the above effect is desired, the Zr content is preferably 0.005% or more.

Ta: 0 to 0.050%
Ta is an element that contributes to the improvement of strength, and although the mechanism is not always clear, it also contributes to the improvement of corrosion resistance, and therefore may be contained if necessary. However, Ta is an expensive element, and its content in a large amount causes an increase in steelmaking cost. Therefore, the Ta content is set to 0.050% or less. The Ta content is preferably 0.040% or less, more preferably 0.030% or less, and even more preferably 0.020% or less. When the above effect is desired, the Ta content is preferably 0.001% or more, more preferably 0.005% or more.

B: 0 to 0.010%
Since B is an element that improves hardenability and enhances strength, it may be contained if necessary. However, even if B is excessively contained, the effect may be saturated and the toughness of the base material and HAZ may decrease. Therefore, the B content is set to 0.010% or less. The B content is preferably 0.0050% or less, more preferably 0.0030% or less, and even more preferably 0.0020% or less. When the above effect is desired, the B content is preferably 0.0003% or more, and more preferably 0.0005% or more.

In the chemical composition of the steel of the present invention, in addition to the above elements, one or more selected from Ca, Mg and REM may be contained in the range shown below for the purpose of deoxidation and control of inclusions. Good. Since these elements are not always essential in steel materials, the lower limit of the content is 0%. The reasons for limiting each element will be described.

Ca: 0 to 0.010%
Ca is an element mainly used for controlling the morphology of sulfides, and may be contained as necessary in order to form fine oxides. However, excessive addition of Ca may impair mechanical properties. Therefore, the Ca content is set to 0.010% or less. The Ca content is preferably 0.005% or less, more preferably 0.003% or less. When the above effect is desired, the Ca content is preferably 0.0005% or more, more preferably 0.001% or more, and further preferably 0.002% or more. ..

Mg: 0 to 0.010%
Mg may be contained if necessary in order to form a fine oxide. However, excessive addition of Mg leads to an increase in steelmaking cost. Therefore, the Mg content is set to 0.010% or less. The Mg content is preferably 0.005% or less, more preferably 0.003% or less. When the above effect is desired, the Mg content is preferably 0.0001% or more, more preferably 0.0003% or more, and further preferably 0.0005% or more. ..

REM: 0-0.010%
REM (rare earth element) is an element mainly used for deoxidation, and may be contained as necessary in order to form a fine oxide. However, excessive addition of REM leads to an increase in steelmaking cost. Therefore, the REM content is set to 0.010% or less. The REM content is preferably 0.005% or less, more preferably 0.003% or less. When the above effect is desired, the REM content is preferably 0.0001% or more, more preferably 0.0003% or more, and further preferably 0.0005% or more. ..

Here, REM is a general term for a total of 17 elements of Sc, Y and lanthanoid, and the content of REM means the total amount of the above elements. Lanthanoids are industrially added in the form of misch metal.

In the chemical composition of the steel material of the present invention, the balance is Fe and impurities. Here, the impurity is a component mixed by raw materials such as ores and scraps and other factors in the industrial production of steel materials, and is allowed as long as it does not adversely affect the steel material according to the present invention. means.

CI: 17.0 to 30.0
The acid resistance corrosion index CI is an index derived to suppress inclusions that are likely to be corrosion origins while forming CuS in steel. Cu and S are required to produce CuS in steel, but the balance of Cu and S is important. In order to significantly improve the corrosion resistance in an acid corrosion environment, the acid corrosion resistance index CI is set to 17.0 to 30.0. The acid resistance corrosion index CI is preferably 18.0 or more, more preferably 19.0 or more, and further preferably 20.0 or more. The acid corrosion resistance index CI is preferably 28.0 or less, more preferably 26.0 or less, and even more preferably 24.0 or less.

The acid resistance corrosion index CI is the ratio of the number of Cu atoms to the number of S atoms, as defined by the following equation (i). That is, Cu / 64 and S / 32 are terms obtained by dividing the contents of Cu and S by the mass number of each element, respectively.
CI = (Cu / 64) / (S / 32) ... (i)

DI: 1.00 to 3.00
The red heat embrittlement index DI is an index for suppressing surface cracks due to Cu segregating at grain boundaries or the surface of ground iron by containing Ni, and is important for ensuring hot workability. If the Ni content is too small with respect to the Cu content, the hot workability deteriorates. On the other hand, it is preferable to reduce the reddish embrittlement index DI in order to ensure hot workability, but if the value is too low, the effect is saturated. Further, when Cu is insufficient, the effect of improving the corrosion resistance in an acid-corroded environment becomes insufficient. From the viewpoint of achieving both hot workability and corrosion resistance, the red hot embrittlement index DI is 1.00 to 3.00. The red embrittlement index DI is preferably 1.25 or more, and more preferably 1.50 or more, from the viewpoint of improving corrosion resistance. Further, from the viewpoint of improving hot workability, the red heat embrittlement index DI is preferably 2.80 or less, and more preferably 2.60 or less.

The red embrittlement index DI is the ratio of the number of Cu atoms to the number of Ni atoms, as defined by Eq. (Ii) below. That is, Cu / 64 and Ni / 59 are terms obtained by dividing the contents of Cu and Ni by the mass number of each element, respectively.
DI = (Cu / 64) / (Ni / 59) ... (ii)

Ceq: 0.100 to 0.220
Ceq is an index showing deterioration of cold workability due to an increase in hardness. If Ceq is excessive, cold workability cannot be ensured. On the other hand, if Ceq is too low, the mechanical properties will be insufficient. Therefore, Ceq is set to 0.100 to 0.220. Ceq is preferably 0.110 or more, and more preferably 0.120 or more. Further, Ceq is preferably 0.210 or less, and more preferably 0.200 or less. Ceq is defined by the following equation (iii).
Ceq = C + Mn / 6 + (Cu + Ni) / 5+ (Cr + Mo + V) / 15 ... (iii)

The element symbols in the above formulas (i) to (iii) represent the content (mass%) of each element contained in the steel material, and if it is not contained, 0 is substituted.

(B) Inclusion The steel material according to the present invention contains MnS and MnS oxide in the steel material. The number density of MnS having a maximum length of 2.0 μm or more is less than 10.0 / mm ² . In addition, the ratio of the number density of MnS oxides having a maximum length of 2.0 μm or more to the number density of MnS having a maximum length of 2.0 μm or more is 0.10 or more.

Since MnS having a maximum length of less than 2.0 μm has almost no effect on the corrosion resistance of the steel material, inclusions having a maximum length of 2.0 μm or more are targeted in the present invention. In the following description, MnS having a maximum length of 2.0 μm or more is simply referred to as MnS, and MnS oxide having a maximum length of 2.0 μm or more is simply referred to as MnS oxide.

As described above, the formation of MnS is unavoidable in the steel material of the present invention. However, MnS becomes a starting point of corrosion and deteriorates corrosion resistance in an acid-corroded environment. Therefore, it is necessary to limit the number density of MnS to less than 10.0 / mm ² . The number density of MnS is preferably 8.0 / mm ² or less, and more preferably 6.0 / mm ² or less.

On the other hand, an extreme reduction in the contents of Mn and S is not preferable in the steel material of the present invention from the viewpoint of improving strength, toughness and corrosion resistance. In order to achieve both of these, it is necessary to detoxify MnS. When MnS is combined with oxygen to become an MnS oxide, it is detoxified and is unlikely to be the starting point of corrosion. Therefore, in the present invention, the ratio of the number density of MnS oxides to the number density of MnS is set to 0.10 or more. The above ratio is preferably 0.12 or more, and more preferably 0.15 or more.

The number density of MnS and the number density of MnS oxide are measured by energy dispersive X-ray analysis (EDS) provided in a scanning electron microscope (SEM). The measurement magnification is 1000 times, and the maximum lengths of MnS and MnS oxides detected in the visual field are measured. Then, the number density is obtained by counting the number of inclusions having a maximum length of 2.0 μm or more and dividing by the visual field area.

The inclusions are identified by EDS, and inclusions having a total content of Mn and S of 90% by mass or more are determined to be MnS, a peak of O is detected, and the total content of Mn, S and O is contained. Inclusions having an amount of 90% by mass or more are judged to be MnS oxides.

(C) Manufacturing Method A method for manufacturing a steel material according to an embodiment of the present invention will be described. The steel material according to the present embodiment includes steel plates, shaped steels, steel pipes, and the like manufactured by hot rolling and, if necessary, cold rolling. Above all, the steel material according to the present invention can be suitably used as a thin plate used for fin materials and the like. Therefore, the thickness of the steel material is preferably 0.5 to 2.5 mm, more preferably 0.7 to 2.3 mm, and even more preferably 1.0 to 1.6 mm.

The steel material according to the present embodiment is produced by melting steel by a conventional method, adjusting the components, hot-rolling the steel pieces obtained by casting, and cold-rolling if necessary. .. In order to promote the formation of CuS, suppress the formation of MnS as much as possible, and inevitably control the ratio of the number densities of MnS and MnS oxides present in the steel material within the above range, before hot rolling. It is important that the heating temperature is relatively low, and specifically, it is preferably 1000 to 1130 ° C.

By lowering the heating temperature before hot rolling, the growth of MnS can be suppressed and the size can be reduced during rolling. Since the finely divided MnS has a relatively large surface area, it easily binds to oxygen and easily becomes an MnS oxide. In order to make the number density of MnS less than 6.0 / mm ² and the ratio of the number density of MnS oxide to MnS 0.12 or more, the heating temperature before hot rolling should be 1080 ° C. or less. More preferred.

Next steps such as cutting or coil winding are added to the hot-rolled steel sheet after hot rolling. At that time, the temperature of the steel sheet drops, but it is desirable that the time from the completion of hot spreading to reaching 400 ° C. is 4 hours or more. After hot rolling, it may be cold rolled to obtain a cold-rolled steel sheet. Further, heat treatment may be performed after cold rolling.

When a steel pipe is manufactured from the obtained steel plate, the steel pipe may be formed into a tubular shape and welded, and for example, a UO steel pipe, an electrosewn steel pipe, a forge welded steel pipe, a spiral steel pipe or the like can be obtained.

Hereinafter, the present invention will be described in more detail with reference to Examples. The conditions in the examples shown below are one-condition examples adopted for confirming the feasibility and effect of the present invention, and the present invention is not limited to this one-condition example. Further, the present invention can adopt various conditions as long as the gist of the present invention is not deviated and the object of the present invention is achieved.

Steels (A1 to 26, B1 to 12) having the chemical compositions shown in Tables 1 and 2 are melted and hot-rolled on the ingots under the conditions shown in Tables 3 and 4, and heat having a thickness of 20 mm is obtained. Manufactured steel sheets. After hot rolling, cooling was performed to simulate winding, and then cold rolling was performed to obtain a cold rolled steel sheet having a thickness of 1.3 mm.

A test piece for SEM observation was cut out from each of the obtained steel sheets, and the number density of inclusions was measured by the EDS provided in the SEM. The measurement magnification shall be 1000 times, the maximum lengths of MnS and MnS oxides detected in the visual field shall be measured, the number of inclusions having a maximum length of 2.0 μm or more, respectively, shall be counted and divided by the visual field area. Then, the number density was calculated.

Furthermore, various performance evaluation tests shown below were carried out using each of the obtained steel sheets.

<Sulfuric acid resistance, sulfuric acid resistance>
A test piece having a plate thickness of 1 mm, a width of 25 mm, and a length of 25 mm was collected from each steel plate from the central portion of the plate thickness and finished by wet # 400 polishing to prepare a test piece for corrosion resistance evaluation. The corrosion resistance was evaluated by a sulfuric acid immersion test and a hydrochloric acid immersion test. In the sulfuric acid immersion test, the test piece was immersed in a 50% sulfuric acid aqueous solution at 70 ° C. for 6 hours, and in the hydrochloric acid immersion test, the test piece was immersed in a 10% hydrochloric acid aqueous solution at 80 ° C. for 5 hours.

Then, the corrosion rate was calculated from the corrosion weight loss of the test piece by the sulfuric acid immersion test and the hydrochloric acid immersion test. In the present embodiment, when the corrosion rate by sulfuric acid immersion test is less than 15.0mg / ^cm 2 / h, it determines that the excellent resistance to sulfuric acid, the corrosion rate with hydrochloric acid immersion test is 10.0 mg / ^cm 2 / When it was h or less, it was judged that the hydrochloric acid resistance was excellent.

<Hot workability>
The surface of the hot-rolled material rolled under the above conditions was visually inspected, and the hot workability was evaluated by valuing the cracked material as x and the non-cracked material as 〇.

<Tensile strength and total elongation>
A tensile test piece having a thickness of 1 mm was prepared in accordance with JIS Z 2241: 2011, and a tensile test was performed to determine the tensile strength and total elongation. Those having a tensile strength of 350 MPa or more were evaluated as ◯, and those having a tensile strength of less than 350 MPa were evaluated as x. The total elongation was used as an index of cold workability, and those with 30% or more were marked with ◯, and those with less than 30% were marked with x.

Tables 5 and 6 summarize the measurement results of the number density of inclusions and the evaluation results of the sulfuric acid immersion test, the hydrochloric acid immersion test, the hot workability and the tensile test.

As shown in Tables 5 and 6, Test Nos. That satisfy all the provisions of the present invention. In 1 to 26, excellent results were obtained in all the performance evaluation tests. On the other hand, Test No. which is a comparative example. In 27 to 36, at least one of sulfuric acid resistance, hydrochloric acid resistance, hot workability and cold workability was deteriorated.

The steel material of the present invention is used for a boiler that burns heavy oil, fossil fuels such as coal, gas fuels such as liquefied natural gas, general wastes such as municipal waste, industrial wastes such as waste oil, plastics and exhaust tires, and sewage sludge. It can be used for smoke exhaust equipment. Specifically, a gas-gas heater composed of a flue duct, a casing, a heat exchanger, and two heat exchangers (heat recovery device and reheater) of smoke exhaust equipment, a desulfurization device, an electrostatic collector, and an attracting blower. , Can be suitably used for basket materials and heat transfer element plates of rotary regeneration type air preheaters.

Claims (4)

The chemical composition is mass%,
C: 0.0010% or more and less than 0.010%,
Si: 0.03 to 0.60%,
Mn: 0.10 to 1.50%,
Cu: 0.05 to 0.50%,
Sb: 0.02 to 0.30%,
Ni: 0.02 to 0.50%,
Cr: 0.02 to 0.09%,
Al: 0.005 to 0.080%,
N: 0.008% or less,
P: 0.025% or less,
S: 0.001 to 0.015%,
O: 0.0005 to 0.0035%,
Mo: 0-0.50%,
W: 0 to 0.50%,
Sn: 0 to 0.30%,
As: 0-0.30%,
Co: 0-0.30%,
Bi: 0-0.010%,
Ti: 0 to 0.050%,
Nb: 0 to 0.10%,
V: 0 to 0.10%,
Zr: 0 to 0.050%,
Ta: 0 to 0.050%,
B: 0 to 0.010%,
Ca: 0-0.010%,
Mg: 0-0.010%,
REM: 0-0.010%,
Remaining: Fe and impurities,
The CI defined by the following equation (i) is 17.0 to 30.0.
The DI defined by the following equation (ii) is 1.00 to 3.00.
The Ceq defined by the following equation (iii) is 0.1000 to 0.220.
For the number density of MnS containing MnS and MnS oxide in the steel material, the number density of MnS having a maximum length of 2.0 μm or more is less than 10.0 / mm ² , and the maximum length is 2.0 μm or more. The ratio of the number density of MnS oxides having a maximum length of 2.0 μm or more is 0.10 or more.
Steel material.
CI = (Cu / 64) / (S / 32) ... (i)
DI = (Cu / 64) / (Ni / 59) ... (ii)
Ceq = C + Mn / 6 + (Cu + Ni) / 5+ (Cr + Mo + V) / 15 ... (iii)
However, the element symbol in the above formula represents the content (mass%) of each element contained in the steel material, and if it is not contained, 0 is substituted. When the chemical composition is mass%,
Mo: 0.01-0.50%,
W: 0.01 to 0.50%,
Sn: 0.01 to 0.30%,
As: 0.01-0.30%,
Co: 0.01 to 0.30%, and Bi: 0.001 to 0.010%,
Contains one or more selected from
The steel material according to claim 1. When the chemical composition is mass%,
Ti: 0.001 to 0.050%,
Nb: 0.001 to 0.10%,
V: 0.005 to 0.10%,
Zr: 0.005 to 0.050%,
Ta: 0.001 to 0.050%, and B: 0.0003 to 0.010%,
Contains one or more selected from
The steel material according to claim 1 or 2. When the chemical composition is mass%,
Ca: 0.0005 to 0.010%,
Mg: 0.0001 to 0.010%, and REM: 0.0001 to 0.010%,
Contains one or more selected from
The steel material according to any one of claims 1 to 3.